1. Field of the Invention
This invention relates to a plain bearing, and more particularly to a plain bearing having a bearing alloy layer of which the inner surface is formed as a textured uneven surface and coated with an overlay.
2. Description of the Related Art
There is known a plain bearing of the type in which a bearing alloy layer is bonded to a back metal and an overlay layer made of a soft material such as a Pbxe2x80x94Sn base alloy is provided on the inner surface of the bearing alloy layer so as to enhance the conformability and the embeddability for a foreign particle. There is also known a plain bearing of the type in which an intermediate layer made of Ni or the like is provided between the bearing alloy layer and the overlay layer so as to prevent components contained in the overlay layer from diffusing into the bearing alloy layer and enhance the bonding property of the overlay layer.
In those types of bearing, when the overlay layer is worn to the extent that the hard intermediate layer is exposed widely, a related shaft would directly slide on the hard intermediate layer and thus the seizure would be liable to occur abruptly. Therefore, there is also known a plain bearing of the type in which a plurality of recess grooves 64 are formed in the circumferential direction on the inner surface of the bearing alloy layer 61 by boring, and an intermediate layer 62 and an overlay layer 63 are further provided on the recess grooves 64, as shown in FIGS. 8 and 9, so that even when the overlay layer 63 is worn, the intermediate layer 62 and/or the bearing alloy layer 61 are not exposed widely but together with the soft overlay layer 63 filling the recess grooves 64 in an intermingled manner for maintaining the anti-seizure property.
However, since the recess grooves 64 formed by boring extend in parallel with each other in the circumferential direction, when a foreign particle X such as a chip or a deterioration product of lubricant is introduced between the shaft and the plain bearing during the operation, the foreign particle is frictionally moved in the circumferential direction while leaving a long scar (or damage) on the surface of the overlay layer. As a result, the anti-seizure property of the bearing is reduced, and the damage may cause the spread of a fatigue crack. Therefore, it has been impossible to provide a plain bearing excellent both in the anti-seizure property and the fatigue resistance.
Accordingly, it is an object of the invention to provide a plain bearing excellent both in the anti-seizure property and the fatigue resistance.
According to a first feature of the invention, there is provided a plain bearing of the type in which protrusions are formed on the inner surface of a bearing alloy layer so that when a foreign particle introduced between sliding surfaces of a shaft and the plain bearing is moved in the circumferential direction in accordance with the rotation of the shaft, the foreign particle runs against a side edge of the protrusions and moves along the side edge, and thereafter leaves the side edge again.
According to this feature of the invention, when a foreign particle is introduced between the sliding surfaces of the shaft and the plain bearing, the foreign particle is moved in the circumferential direction while being embedded in the soft overlay layer by the press of the rotating shaft. After that, the foreign particle runs against a side edge of one of the protrusions and moves along the side edge while being pressed against the side edge. This pressing force promotes the embedding of the foreign particle into the overlay layer. Thus, the foreign particle does not bite the side edge of the protrusions.
Moreover, in order to move the foreign particle along a side edge of the protrusions, it is necessary that the side edge is oblique at about 60xc2x0 relating to the circumferential direction. While the side edge having a small oblique angle relating to the circumferential direction causes the foreign particle to move along the side edge easily, the embedding function of the plain bearing is reduced. Thus, it is desirable that the side edge is oblique at from 5xc2x0 to 60xc2x0, preferably from 10xc2x0 to 50xc2x0.
The bearing alloy may be made of a Cu alloy or an Al alloy, for representative examples. The overlay layer may be made of one selected from the group of Pb, a Pb alloy, Sn, a Sn alloy or a known sliding resin such as PTFE and PFA, for example.
Moreover, the bearing alloy layer may be directly covered with the overlay layer. Alternatively, the intermediate layer such as Ni and Ag may be provided between the bearing alloy layer and the overlay layer so as to enhance the bonding strength and prevent components contained in the overlay layer from diffusing into the bearing alloy layer, as a second feature. The intermediate layer may be made of one selected from the group of Ni, Ag, a Ni alloy, Cu, a Cu alloy, Co, a Co alloy, Sn, a Sn alloy, a Ag alloy, Zn and a Zn alloy, for example.
The overlay may be made of a lead-tin alloy, for a representative example. However, the overlay layer may be made of other soft material, such as a resin and a tin base alloy.
In the first feature of the invention, protrusions are arranged so that when a foreign particle introduced between sliding surfaces of a shaft and a plain bearing is moved in the circumferential direction in accordance with the rotation of the shaft, the foreign particle runs against a side edge of the protrusions and moves along the side edge, and thereafter leaves the side edge again, whereby there is provided a plain bearing excellent in the anti-seizure property and the embeddability for a foreign particle.